xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/IPO/ConstantMerge.cpp (revision 6966ac055c3b7a39266fb982493330df7a097997)
1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file defines the interface to a pass that merges duplicate global
10 // constants together into a single constant that is shared.  This is useful
11 // because some passes (ie TraceValues) insert a lot of string constants into
12 // the program, regardless of whether or not an existing string is available.
13 //
14 // Algorithm: ConstantMerge is designed to build up a map of available constants
15 // and eliminate duplicates when it is initialized.
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #include "llvm/Transforms/IPO/ConstantMerge.h"
20 #include "llvm/ADT/DenseMap.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DataLayout.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/GlobalValue.h"
28 #include "llvm/IR/GlobalVariable.h"
29 #include "llvm/IR/LLVMContext.h"
30 #include "llvm/IR/Module.h"
31 #include "llvm/Pass.h"
32 #include "llvm/Support/Casting.h"
33 #include "llvm/Transforms/IPO.h"
34 #include <algorithm>
35 #include <cassert>
36 #include <utility>
37 
38 using namespace llvm;
39 
40 #define DEBUG_TYPE "constmerge"
41 
42 STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
43 
44 /// Find values that are marked as llvm.used.
45 static void FindUsedValues(GlobalVariable *LLVMUsed,
46                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
47   if (!LLVMUsed) return;
48   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
49 
50   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
51     Value *Operand = Inits->getOperand(i)->stripPointerCastsNoFollowAliases();
52     GlobalValue *GV = cast<GlobalValue>(Operand);
53     UsedValues.insert(GV);
54   }
55 }
56 
57 // True if A is better than B.
58 static bool IsBetterCanonical(const GlobalVariable &A,
59                               const GlobalVariable &B) {
60   if (!A.hasLocalLinkage() && B.hasLocalLinkage())
61     return true;
62 
63   if (A.hasLocalLinkage() && !B.hasLocalLinkage())
64     return false;
65 
66   return A.hasGlobalUnnamedAddr();
67 }
68 
69 static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
70   SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
71   GV->getAllMetadata(MDs);
72   for (const auto &V : MDs)
73     if (V.first != LLVMContext::MD_dbg)
74       return true;
75   return false;
76 }
77 
78 static void copyDebugLocMetadata(const GlobalVariable *From,
79                                  GlobalVariable *To) {
80   SmallVector<DIGlobalVariableExpression *, 1> MDs;
81   From->getDebugInfo(MDs);
82   for (auto MD : MDs)
83     To->addDebugInfo(MD);
84 }
85 
86 static unsigned getAlignment(GlobalVariable *GV) {
87   unsigned Align = GV->getAlignment();
88   if (Align)
89     return Align;
90   return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
91 }
92 
93 static bool
94 isUnmergeableGlobal(GlobalVariable *GV,
95                     const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
96   // Only process constants with initializers in the default address space.
97   return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
98          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
99          // Don't touch values marked with attribute(used).
100          UsedGlobals.count(GV);
101 }
102 
103 enum class CanMerge { No, Yes };
104 static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
105   if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
106     return CanMerge::No;
107   if (hasMetadataOtherThanDebugLoc(Old))
108     return CanMerge::No;
109   assert(!hasMetadataOtherThanDebugLoc(New));
110   if (!Old->hasGlobalUnnamedAddr())
111     New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
112   return CanMerge::Yes;
113 }
114 
115 static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
116   Constant *NewConstant = New;
117 
118   LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
119                     << New->getName() << "\n");
120 
121   // Bump the alignment if necessary.
122   if (Old->getAlignment() || New->getAlignment())
123     New->setAlignment(std::max(getAlignment(Old), getAlignment(New)));
124 
125   copyDebugLocMetadata(Old, New);
126   Old->replaceAllUsesWith(NewConstant);
127 
128   // Delete the global value from the module.
129   assert(Old->hasLocalLinkage() &&
130          "Refusing to delete an externally visible global variable.");
131   Old->eraseFromParent();
132 }
133 
134 static bool mergeConstants(Module &M) {
135   // Find all the globals that are marked "used".  These cannot be merged.
136   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
137   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
138   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
139 
140   // Map unique constants to globals.
141   DenseMap<Constant *, GlobalVariable *> CMap;
142 
143   SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
144       SameContentReplacements;
145 
146   size_t ChangesMade = 0;
147   size_t OldChangesMade = 0;
148 
149   // Iterate constant merging while we are still making progress.  Merging two
150   // constants together may allow us to merge other constants together if the
151   // second level constants have initializers which point to the globals that
152   // were just merged.
153   while (true) {
154     // Find the canonical constants others will be merged with.
155     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
156          GVI != E; ) {
157       GlobalVariable *GV = &*GVI++;
158 
159       // If this GV is dead, remove it.
160       GV->removeDeadConstantUsers();
161       if (GV->use_empty() && GV->hasLocalLinkage()) {
162         GV->eraseFromParent();
163         ++ChangesMade;
164         continue;
165       }
166 
167       if (isUnmergeableGlobal(GV, UsedGlobals))
168         continue;
169 
170       // This transformation is legal for weak ODR globals in the sense it
171       // doesn't change semantics, but we really don't want to perform it
172       // anyway; it's likely to pessimize code generation, and some tools
173       // (like the Darwin linker in cases involving CFString) don't expect it.
174       if (GV->isWeakForLinker())
175         continue;
176 
177       // Don't touch globals with metadata other then !dbg.
178       if (hasMetadataOtherThanDebugLoc(GV))
179         continue;
180 
181       Constant *Init = GV->getInitializer();
182 
183       // Check to see if the initializer is already known.
184       GlobalVariable *&Slot = CMap[Init];
185 
186       // If this is the first constant we find or if the old one is local,
187       // replace with the current one. If the current is externally visible
188       // it cannot be replace, but can be the canonical constant we merge with.
189       bool FirstConstantFound = !Slot;
190       if (FirstConstantFound || IsBetterCanonical(*GV, *Slot)) {
191         Slot = GV;
192         LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV->getName()
193                           << (FirstConstantFound ? "\n" : " (updated)\n"));
194       }
195     }
196 
197     // Identify all globals that can be merged together, filling in the
198     // SameContentReplacements vector. We cannot do the replacement in this pass
199     // because doing so may cause initializers of other globals to be rewritten,
200     // invalidating the Constant* pointers in CMap.
201     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
202          GVI != E; ) {
203       GlobalVariable *GV = &*GVI++;
204 
205       if (isUnmergeableGlobal(GV, UsedGlobals))
206         continue;
207 
208       // We can only replace constant with local linkage.
209       if (!GV->hasLocalLinkage())
210         continue;
211 
212       Constant *Init = GV->getInitializer();
213 
214       // Check to see if the initializer is already known.
215       auto Found = CMap.find(Init);
216       if (Found == CMap.end())
217         continue;
218 
219       GlobalVariable *Slot = Found->second;
220       if (Slot == GV)
221         continue;
222 
223       if (makeMergeable(GV, Slot) == CanMerge::No)
224         continue;
225 
226       // Make all uses of the duplicate constant use the canonical version.
227       LLVM_DEBUG(dbgs() << "Will replace: @" << GV->getName() << " -> @"
228                         << Slot->getName() << "\n");
229       SameContentReplacements.push_back(std::make_pair(GV, Slot));
230     }
231 
232     // Now that we have figured out which replacements must be made, do them all
233     // now.  This avoid invalidating the pointers in CMap, which are unneeded
234     // now.
235     for (unsigned i = 0, e = SameContentReplacements.size(); i != e; ++i) {
236       GlobalVariable *Old = SameContentReplacements[i].first;
237       GlobalVariable *New = SameContentReplacements[i].second;
238       replace(M, Old, New);
239       ++ChangesMade;
240       ++NumIdenticalMerged;
241     }
242 
243     if (ChangesMade == OldChangesMade)
244       break;
245     OldChangesMade = ChangesMade;
246 
247     SameContentReplacements.clear();
248     CMap.clear();
249   }
250 
251   return ChangesMade;
252 }
253 
254 PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
255   if (!mergeConstants(M))
256     return PreservedAnalyses::all();
257   return PreservedAnalyses::none();
258 }
259 
260 namespace {
261 
262 struct ConstantMergeLegacyPass : public ModulePass {
263   static char ID; // Pass identification, replacement for typeid
264 
265   ConstantMergeLegacyPass() : ModulePass(ID) {
266     initializeConstantMergeLegacyPassPass(*PassRegistry::getPassRegistry());
267   }
268 
269   // For this pass, process all of the globals in the module, eliminating
270   // duplicate constants.
271   bool runOnModule(Module &M) override {
272     if (skipModule(M))
273       return false;
274     return mergeConstants(M);
275   }
276 };
277 
278 } // end anonymous namespace
279 
280 char ConstantMergeLegacyPass::ID = 0;
281 
282 INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
283                 "Merge Duplicate Global Constants", false, false)
284 
285 ModulePass *llvm::createConstantMergePass() {
286   return new ConstantMergeLegacyPass();
287 }
288